U.S. Pat. No. 4,005,330 to John M. Anderson and Homer H. Glascock, Jr. describes induction ionized fluorescent lamps wherein a solenoidal electric field is produced by induction through an annular magnetic core which is external to, yet centrally disposed within, a substantially globular lamp envelope. The magnetic core passes through a channel in the lamp envelope to link a working gas there-within. Fluorescent lamps constructed in accordance with the teachings of that patent may be physically and electrically compatible with screw-base incandescent lamps, yet provide operating efficiencies comparable to those of conventional fluorescent lamps. The specification of U.S. Pat. No. 4,005,330 is incorporated herein by reference, as background material for this invention.
The maximum operating power level usable in solenoidal electric field, fluorescent lamps has been found to be limited by the thermal characteristics of the magnetic cores, which are typically ferrites. Saturation magnetic flux density in conventional ferrite cores has, for example, been found to decrease rapidly as the core temperature approaches a limit of approximately 125.degree. C. Magnetic losses within the ferrite also tend to increase with increased temperature. Thus, for a lamp of given physical dimensions, ferrite temperature effectively determines the maximum permissible operating power level. It is, thus, important to keep ferrite temperatures from reaching too high a value.
The regions of the envelope directly adjacent the magnetic core in the lamps of U.S. Pat. No. 4,005,330, that is, the header and tunnel regions, are typically coated with ultraviolet-to-visible light converting phosphors of the type which are normally utilized in conventional fluorescent lamps. The ultraviolet flux density and temperature at the header and tunnel regions is generally considerably higher than at other portions of the lamp envelope; a condition which tends to result in poor lumen maintenance for phosphors deposited in those regions.